Damascene copper (Cu) metallization is the current interconnect metallization of choice for 20, 14 and 10 nm technology nodes and is likely to be method of choice for future 7, 5 and 3 nm nodes as well because copper has a lower resistivity in comparison to other low-cost alternative materials, such as aluminum (Al). A copper damascene process relies on a diffusion barrier to prevent copper from diffusing into an Inter Layer Dielectric (ILD). The diffusion barrier is commonly formed from tantalum nitride (TaN) or titanium nitride (TiN). Copper cannot diffuse through the barrier, and the metals forming the barrier do not diffuse into the ILD. While diffusion barriers formed by deposition are successful in keeping copper from diffusing into an ILD, the diffusion barriers also reduce the amount of copper that would be present in the cross section of an interconnect line or a via if no barrier was present.
Additionally, a metal liner is used to promote adhesion between copper and the diffusion barrier. Accordingly, to prevent copper diffusion into an ILD and to promote copper adhesion, diffusion barriers and metal liners must have a minimum thickness, which causes a further net reduction in copper cross-sectional area and an increase in the overall resistivity of interconnect lines and vias.
As interconnect pitch scales smaller, the copper cross-sectional area of an interconnect line further decreases and reduces overall line width. The metal half-pitch requirements are often met by scaling the amount of copper present along the width of an interconnect at a first scaling rate while the barrier plus liner width is scaled at a second, lesser scaling rate, which further results in a net reduction in copper cross-sectional area.